> ## Documentation Index
> Fetch the complete documentation index at: https://docs.valar.space/llms.txt
> Use this file to discover all available pages before exploring further.
# Force Model
> How VALAR computes drag and solar radiation pressure forces, and what triggers attitude-dependent force modeling for your spacecraft
> For the complete documentation index, see [llms.txt](/llms.txt).
VALAR computes non-gravitational forces — atmospheric drag and solar radiation pressure (SRP) — using one of two modes per spacecraft per analysis: an **isotropic** sphere model and an **attitude-dependent** per-surface model. The mode is selected automatically for each analysis based on what the spacecraft has configured. There is no toggle and no per-analysis decision.
***
## Isotropic Mode
The isotropic model treats the spacecraft as a sphere with two operator-typed cross-sections, a single drag coefficient, and a single reflectivity coefficient. Drag and SRP accelerations are independent of spacecraft orientation.
* **Drag area (A\_drag):** operator-typed cross-section for atmospheric drag (m²)
* **SRP area (A\_SRP):** operator-typed cross-section for solar radiation pressure (m²)
* **Drag coefficient (Cd):** scalar
* **Reflectivity coefficient (Cr):** scalar, range `[1, 2]`
This is the default for spacecraft that do not have both a geometry and an attitude profile configured.
***
## Attitude-Dependent Mode
The attitude-dependent model computes drag and SRP **per surface** using the spacecraft's panel geometry and the time-varying spacecraft attitude. For each numerical integration step:
* The spacecraft attitude is evaluated from the configured attitude profile
* Each surface contributes drag and SRP based on its outward-facing normal in the inertial frame, the relative velocity vector (drag) or Sun direction (SRP), the surface area, and its optical properties (absorption α, specular reflection ρs, and the implied diffuse reflection coefficient `1 − α − ρs`)
* Surfaces that face away from the velocity vector or the Sun contribute zero force on the corresponding term
This produces a force that varies with attitude, which is essential for spacecraft whose cross-section and reflective characteristics depend strongly on orientation (e.g., spacecraft with deployable solar arrays).
***
## How VALAR Chooses the Mode
The selection is automatic per spacecraft:
| Spacecraft configuration | Force model |
| ------------------------------------------------- | ------------------ |
| Geometry **and** attitude profile both configured | Attitude-dependent |
| Either geometry or attitude profile missing | Isotropic |
There is no toggle in the UI, no per-analysis override, and no API field that overrides the eligibility rule. Configuring an attitude profile for a spacecraft that already has geometry automatically upgrades all subsequent numerically-propagated analyses to attitude-dependent.
Geometry is configured on the spacecraft create/edit dialog (panel pairs and panel normal). Attitude profiles are configured under the spacecraft's Attitude section. See [What you need to configure](#what-you-need-to-configure) below.
***
## Where the Mode Applies
Attitude-dependent force modeling applies to every analysis that uses VALAR's numerical propagator:
* **Orbit determination** — both the propagation inside the least-squares filter and any subsequent state propagation
* **Conjunction assessment** — primary and secondary state propagation
* **Pass computation** — propagation across the access window
* **Orbital events** — propagation for event detection
* **Collision avoidance manoeuvre (CAM) optimisation** — propagation across the manoeuvre window
* **Numerical manoeuvre planning** — propagation across the plan horizon
### Exception: Long-Term Forecast
The **long-term forecast** (which uses a semi-analytical propagator) **always uses the isotropic model**, regardless of spacecraft configuration. Semi-analytical theory operates on mean elements and does not resolve per-surface, attitude-dependent forces. If a long-term-decay or lifetime forecast disagrees with the equivalent numerical propagation, the difference can include the contribution from per-surface attitude-resolved drag and SRP.
***
## Orbit Determination Parameters
Orbit determination estimates the same parameters in both modes:
* A scalar **drag coefficient (Cd)**
* A scalar **reflectivity coefficient (Cr)**
Switching a spacecraft from isotropic to attitude-dependent (by configuring an attitude profile after geometry is already set, or vice versa) does not change which parameters OD estimates. The interpretation of those parameters changes — Cd and Cr act as scaling factors over the per-surface contributions in attitude-dependent mode — but the OD setup, residuals, and convergence reporting remain consistent.
***
## What You Need to Configure
To upgrade a spacecraft to attitude-dependent force modeling, both pieces must be present:
1. **Spacecraft geometry** — the number of panel pairs, the panel normal vector, and per-surface optical properties (absorption α, specular reflection ρs). See [Spacecraft Management](/features/spacecraft-management).
2. **Attitude profile** — an attitude law, an attitude ephemeris (AEM), or another supported attitude source attached to the spacecraft. See [Attitude](/features/attitude/overview).
If either is missing, every analysis for that spacecraft runs on the isotropic model.
***
Configure geometry and optical properties
Configure an attitude profile
Estimate Cd and Cr from tracking data
Understand coordinate systems